British diplomats are pressing for an explanation for the early release of a US informant who helped train the terrorists behind London's 7 July bombings.

Mohammed Junaid Babar, 35, was quietly released by a New York judge two months ago after serving only four and a half years of a possible 70-year sentence, raising comparisons with the lenient treatment received by the Lockerbie bomber Abdelbaset al-Megrahi.

The Foreign Office said: "Mohammed Junaid Babar's release has caused pain and distress to those who lost family and friends on 7 July 2005.

"The prosecution and sentencing of Babar was a matter for the US authorities and their independent judicial system. The families have a right to an explanation of what lay behind this decision and we will be pressing the US authorities to give them one."

The foreign secretary, William Hague, is expected to face pressure in the House of Commons over the affair after MPs from across the political spectrum came forward on Monday to question the decision. After a Guardian investigation into the case, it is understood that Douglas Alexander, the shadow foreign secretary, will be putting a written parliamentary question to Hague to ask what discussions he had with US authorities about Babar's release.

The issue was also raised in the continuing coroner's inquiry into the bombings in 2005. Speaking on behalf of bereaved families, barrister Caoilfhionn Gallagher told the coroner, Lady Justice Hallett, that her clients wanted to ensure that UK security services "were not aware of any basis for the suggestion that Babar had been an informant for the authorities for any country prior to his detention" in 2004.

The inquest is this week expected to start addressing whether the 7 July attacks could have been prevented.

After the FBI arrested Babar, in April 2004, he confessed to US prosecutors that he had set up a training camp in north-west Pakistan in 2003 and had provided lodgings and transport for nearly a dozen radicals. Among these was Mohammad Sidique Khan, the Yorkshire suicide bomber who along with three fellow terrorists killed 52 people and injured more than 750 on London's transport system in 2005. The camp taught how to fire machine guns and rocket-propelled grenades, and taught bomb-making techniques.

Babar pleaded guilty and turned state witness for the US government, and was released early. He was praised by the sentencing judge and US prosecutors, who described his work as "exceptional" and "extraordinary". However, politicians on this side of the Atlantic want questions asked about the release.

The Conservative former shadow home secretary David Davis MP said Babar was "no hero" and that the US authorities should offer the survivors and those bereaved by the London bombings a "proper explanation" for why Babar had served such a short sentence.

Davis said: "It seems to me that the US authorities have got some questions to answer. On the one hand, they understandably demand that we explain how al-Megrahi came to be released. But here they are, releasing after less than five years a man who knew the London bombers but failed to provide the information that could have prevented 7/7 happening.

"This man, Babar, is no hero. He is a terrorist, and the relatives of the dead and wounded from 7/7 should reasonably expect a proper explanation from the American authorities."

Patrick Mercer, a fellow Tory and former chairman of the counter-terrorism subcommittee, described the release as a disgrace and called on Hague to intervene. "This is the only man to be brought to justice in connection with the 7/7 atrocities. For him to be treated so lightly by the US courts will cause enormous resentment in this country.

"I have no doubt that the foreign secretary will have some very serious questions to ask of his American counterparts," Mercer said.